Ankle-Foot Prosthesis for Automatic Adaptation to Sloped Walking Surfaces

ABSTRACT

An ankle-foot prosthesis includes a foot plate, an ankle frame attached to the foot plate, a yoke pivotally connected to the ankle frame and including a member for attaching to a leg, a damper having a first end connected to the yoke and a second end connected to the ankle frame, and a control mechanism for switching the damper between low and high settings.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority based on prior two (2) U.S.Provisional Applications Ser. No. 61/703,799, filed Sep. 21, 2012, andSer. No. 61/851,740, filed Mar. 13, 2013, both hereby incorporatedherein in their entirety by reference. The present application isfurther related to International Application No. PCT/US2007/022208,filed Oct. 17, 2007 (WO 2008/048658, Apr. 24, 2008) (U.S. applicationSer. No. 12/311,818, filed Apr. 13, 2009, Published as US 2010/0185301,on Jul. 22, 2010), U.S. application Ser. No. 12/462,056, filed Jul. 28,2009 (Published as US 2010/0030343, on Feb. 4, 2010), U.S. applicationSer. No. 13/066,361, filed Apr. 12, 2011 (Published as US 2012/0016493,on Jan. 19, 2012), U.S. application Ser. No. 13/374,881, filed Jan. 20,2012 (Published as US 2013/0006386, on Jan. 3, 2013), and InternationalApplication No. PCT/US2011/000675, filed Apr. 4, 2011 (WO 2011/129892,Oct. 20, 2011), all of which are hereby incorporated herein in theirentirety by reference.

FIELD AND BACKGROUND OF THE INVENTION

The present invention is generally directed to prosthetic and orthoticdevices, and more particularly to an ankle-foot prosthesis for automaticadaptation to level, as well as sloped walking surfaces. Even moreparticularly, the invention is directed to a device or system for use bylower limb amputees to more easily and safely walk over a variety ofsloped terrain, as well as to provide more stability during standing andswaying tasks.

Most currently available prosthetic ankle devices are spring-likestructures that operate about one equilibrium point (i.e., one restingangle). These systems can work nicely on level terrain but causeinstabilities when lower limb prosthesis users walk on sloped surfaces.Many systems have been described that use hydraulic dampers and/orvariations of damping to adjust the properties of the prosthesis (Mauch,1958—U.S. Pat. No. 2,843,853; Koniuk, 2002—U.S. Pat. No. 6,443,993;Moser et al, 2011—U.S. Pat. No. 7,985,265), including the use ofmicroprocessor-control to adjust damping properties. The inherentproblem with damping control of the ankle is the associated loss ofenergy that occurs. One system exists that uses a motor to change theequilibrium point of a spring-like prosthetic foot (Jonsson et al,2011—U.S. Pat. No. 8,048,172). However, this system requires multiplesteps on a new terrain before it is able to adapt to the new slope. Amore desirable system would adapt to different sloped surfaces on eachand every step of walking. Lastly, powered ankle-foot systems are beingdeveloped (Hugh Herr, Massachusetts Institute of Technology; ThomasSugar, Arizona State University; Michael Goldfarb, VanderbiltUniversity). These systems all actively push the prosthesis user with amotor during various times in the gait cycle and require large powersources, e.g., heavy batteries and motors. The only currently availablesystem on the market (iWalk BiOM) is expensive, making it impracticalfor the majority of lower limb prosthesis users. Also, the high powerrequirements necessitate carrying additional batteries and frequentcharging of batteries.

ASPECTS OF THE INVENTION

The present disclosure is directed to various aspects of the presentinvention.

One aspect of the present invention is to provide an ankle-footprosthesis that allows a user to have a more natural, and thus morecomfortable gait.

Another aspect of the present invention is to provide an ankle-footprosthesis that is more energy-efficient when used for walking or othergait.

Another aspect of the present invention is to provide an ankle-footprosthesis that is simple in design and construction and, thus, usesfewer parts or components, and requires no or low maintenance.

Another aspect of the present invention is to provide an ankle-footprosthesis that is compact and more durable than, for example, thoseusing multitude of mechanical parts leading to a higher rate of failure.

Another aspect of the present invention is provide an ankle-footprosthesis that resists or prevents undesirable backward swing, whichcould lead to imbalance or injury.

Another aspect of the present invention is to provide an ankle-footprosthesis that is quieter, light-weight, and less clumsy to use, andthus more user-friendly.

Another aspect of the present invention is to provide an ankle-footprosthesis that automatically adapts to different sloped walkingsurfaces on every step of walking.

Another aspect of the present invention is to provide an ankle-footprosthesis that can easily switch into a stable mode for standing orswaying, for example, when washing the dishes.

Another aspect of the present invention is to provide an ankle-footprosthesis, which includes a foot plate, an ankle frame attached to thefoot plate, a yoke pivotally connected to the ankle frame and includinga member for attaching to a leg, a damper having a first end connectedto the yoke and a second end connected to the ankle frame, and a controlmechanism for switching the damper between low and high settings.

Another aspect of the present invention is to provide an ankle-footprosthesis, which includes a foot plate, an ankle frame attached to thefoot plate and including anterior and posterior portions and an apexportion, a yoke pivotally connected to the apex portion of the ankleframe and including a member for attaching to a leg, a hydraulic damperhaving a first end pivotally connected to the yoke and a second endconnected to the posterior portion of the ankle frame; a spring disposedin parallel to the damper, and a control mechanism for controllingextension and compression of the damper.

Another aspect of the present invention is to provide a method of usingan ankle-foot prosthesis by an amputee, which includes a) providing anankle-foot prosthesis including i) a foot plate, ii) an ankle frameattached to the foot plate, iii) a yoke pivotally connected to the ankleframe and including a member for attaching to a leg, iv) a damper havinga first end connected to the yoke and a second end connected to theankle frame, and v) a control mechanism for switching the damper betweenlow and high settings to selectively control extension, compression, orboth extension and compression thereof; b) attaching the ankle-footprosthesis to a lower limb of the amputee; c) allowing the amputee toambulate for at least one gait cycle, wherein the gait cycle includes i)the ankle-foot prosthesis in an initial neutral position to a firstplantarflexion position such that the foot plate is substantially flaton a walking surface, and ii) the ankle-foot prosthesis in a toe-offplantarflexion position; d) switching the damper to the high extensionsetting substantially at the first plantarflexion position; and e)switching the damper to the low extension setting substantially at thetoe-off plantarflexion position.

In summary, the present invention is directed to a prosthetic ankle-footdevice that can automatically adapt its function for walking ondifferent sloped surfaces, allowing its user to walk on these surfaceswith more stability and confidence. The invention also provides a stablemode for standing and swaying tasks (e.g., washing the dishes).

BRIEF DESCRIPTION OF THE DRAWINGS

One of the above and other aspects, novel features and advantages of thepresent invention will become apparent from the following detaileddescription of the non-limiting preferred embodiment(s) of invention,illustrated in the accompanying drawings, wherein:

FIG. 1 is a perspective view of a preferred embodiment of the ankle-footprosthesis in accordance with the present invention;

FIG. 2 illustrates a blank timing plot for the ankle-foot prosthesis ofthe present invention, which can be used to create any gait cycle thatbegins with prosthetic heel contact (HC) and continues until the next HC(100% of the gait cycle);

FIG. 3 illustrates a plot of the theoretical vertical load on theprosthesis;

FIG. 4 illustrates a plot of the load on spring/damper combination atthe start of stance phase, the heel of the prosthesis making contactwith the surface, placing a compressive load on the spring/dampercombination;

FIG. 5 illustrates a plot of the damping values in each direction forthe damper during the gait cycle;

FIG. 6 illustrates a plot of the cylinder (damper) length;

FIG. 7 shows three hydraulic circuit symbols used for fluid circuitschematics shown in FIGS. 8-16; and

FIGS. 8-16 disclose various preferred embodiments of the fluid controlcircuit (FCC) used in the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) OF THE INVENTION

Referring to FIG. 1, a preferred embodiment of the ankle-foot prosthesisAFP will be described. As shown, a generally pyramid-like attachmentpart 10, consistent with standard endoskeletal componentry inprosthetics, is provided at the top of a yoke 12, on the opposite endsof which are holes drilled for front and rear pivotal attachments 14 and16, respectively. The rear pivot 16 attaches to one end 17 of apreferably microprocessor controlled damper device 18 (to be describedin more detail below). A neutralizing spring 20 is connected in parallelto the damper 18, such that its length change is equal to that of thedamper.

The damper device 18 attaches on its other end 19 to an ankle frame 22,which has a yoke opening 24 and holes drilled at its posterior end 26 topivotally attach to the damper 18 using a shaft 28. The “ankle” of thedevice AFP is a shaft 30 connecting the yoke 12 with the apex 29 of theankle frame 22.

The ankle frame 22 attaches with one or more bolts (or other suitablefixation means) to the rear portion 32 of a flexible, yet deflectablerigid foot plate 34. The anterior end 36 of the ankle frame 22 includesa follower or upwardly inclined surface 38 that limits the deflection ofthe foot plate 34, such that the ankle-foot device AFP will take abiomimetic ankle-foot roll-over shape during walking. The geometry ofthe surface 38 is such that it provides the correct roll-over shape whenthe “ankle” is locked into a plantarflexed angle at the time of footflat of walking, i.e., an angle of about 10 to 15 degrees.

The damper 18 is designed to have different values for compression andextension damping that can be controlled by using a suitablemicroprocessor (not shown). Specifically, the microprocessor would havethe capability to variably manage the timing for opening and closing thevalves and the variable restriction element, shown in FIG. 7 anddescribed below in more detail. For walking, the compression damping isset to a very low level and is unchanged throughout the gait cycle. Theextension damping for walking is set to a very high level at thebeginning of the gait cycle and changes to a very low level damping atthe time of toe-off (which must be sensed using one or more sensors offorce, acceleration, or other properties). The extension damping canremain at a low level of damping for at least the time needed to returnthe ankle to a neutral or dorsiflexed position for swing phase and atmost the time to the next foot flat event of the prosthesis. Forstanding, both compression and extension damping levels can becontrolled to be very high, providing a flatter effective shape andincreasing the stability of the prosthesis user.

For a normal gait cycle, the heel of the system, shown in FIG. 1, willcontact the surface and the foot will “find the surface” under a lowstiffness of the neutralizing spring 20. The compression damping is lowso the ankle reacts primarily in this phase of gait to the neutralizingspring 20. The compression damping could be altered for differentpatients, but would be static throughout the gait cycle once set by theprosthetist or the user. After foot flat, the ankle is at a maximallyplantarflexed position and would normally start to dorsiflex. In thisinvention, the extension damping would be very high, essentially lockingthe ankle in a plantarflexed position. As the person rolls over thedevice, the flexible footplate 34 flexes up to the follower 38,producing a biomimetic ankle-foot roll-over shape. After the oppositefoot contacts the ground, energy is returned from the flexible footplate34 and the ankle goes into late stance plantarflexion (the angle atwhich it was set to at foot flat). As the prosthetic device leaves thewalking surface (toe-off), the extension damping switches a very lowlevel, allowing the neutralizing spring 20 to bring the ankle back to aneutral or slightly dorsiflexed position, which allows for toe clearanceduring the swing phase. The damping level needs to be low enough toallow the ankle to return to neutral during the first third or half ofthe swing phase. After the ankle has returned to neutral or a slightlydorsiflexed position and before the prosthesis gets to foot flat on thenext cycle, the extension damping should shift to a very high level forthe next cycle.

The operation of the ankle in the manner described above, allows thefoot to “find the surface” during walking. For uphill walking, the footfinds the surface in a more dorsiflexed position compared with that forlevel walking and thus the equilibrium point of the ankle is set in moredorsiflexion. For downhill walking, the foot finds the surface in a moreplantarflexed position compared with that for level walking. In thisway, the ankle-foot device automatically adapts to different terrain oneach and every step of walking. Also, the control mechanism for theankle would be relatively simple in that it only changes the extensiondamping of the damper 18 between two levels during walking. The controlmechanism also needs to determine when the ankle-foot system is in“walking” and “standing” modes and switch its behavior. For “standing”mode, the damping for both compression and extension of the damper 18should be set to very high, as mentioned earlier.

Referring to FIGS. 2-6, various timing plots for the ankle-footprosthesis AFP of the invention will now be described. The blank plot ofFIG. 2 can be used to create any gait cycle that begins with prostheticheel contact (HC) and continues until the next HC (100% of the gaitcycle). For the first 10% of the gait cycle, both feet are on the groundas weight transitions from the opposite limb to the prosthetic limb. Atapproximately 10%, the opposite toe is lifted from the floor (oppositetoe-off or OTO). When the opposite toe is off the ground, only theprosthetic foot is in contact with the walking surface and supportingthe weight of the body, so this is considered prostheticsingle-limb-support (40% of the gait cycle). This also corresponds tothe period of highest load on the prosthetic limb as the user rolls overtheir foot. At approximately 50% of the gait cycle, the opposite heelcontacts the surface (OHC) and weight begins to transition from theprosthetic limb to the sound limb. At approximately 60% of the gaitcycle, the prosthetic limb is lifted off the surface (toe-off or TO) andthe remainder of the gait cycle is sound side single limb support as theprosthetic limb swings through. (These numbers are approximate.) Modernconventional usages are changing to use Initial Contact, instead of HeelContact, because the heel is not always the first part of the foot tocome into contact with the walking surface (such as persons withdrop-foot syndrome or equinus deformity), but for this invention the twoshould be equivalent.

FIG. 3 illustrates a plot of the theoretical vertical load on theprosthesis. The plot shows the standard double-hump shape of thevertical component of the ground reaction force vector from standardgait analyses. The first peak corresponds to load acceptance on theprosthetic side, where the prosthesis is used to brake the descent ofthe body center of mass. The second peak occurs as the user pushes offof their prosthetic side and begins to transition onto their sound side(and occurs around opposite heel contact (OHC). The vertical load dropsto 0 as the toe leaves the surface (toe-off, TO) and remains therethrough swing phase. During single limb support, the vertical load canreach levels that are greater than body weight. For slow walking, thepeak load could be 1.1×BW (Body Weight), whereas for fast walking orsudden stumbles the peak load could be upwards of 1.5×BW (Body Weight).

FIG. 4 illustrates a plot of the load on the spring/damper combinationat the start of stance phase, the heel of the prosthesis making contactwith the surface, placing a compressive load on the spring/dampercombination. This load continues until the foot is resting flat againstthe walking surface. At that time, the user begins to roll over theirprosthetic foot. The damper is set to a high extension damping level, soit does not extend, thus the spring becomes an internal stress and whenthe user starts to roll over their foot, the compressive load veryrapidly switches to a small tensile load and then the tensile loadgradually increases as the user rolls over the foot. Near the end ofstance phase, the user has rolled over the prosthesis and begins to liftthe foot from the ground, reducing the tensile force on thespring-damper combination, until the toe is lifted from the surface. Atthis point the spring is still applying an internal force within thesystem but is unable to actuate motion because the damper is still in ahigh extension damping state and resists the spring. When the load isremoved, the hydraulic cutoff valve (fluid equivalent of a switch) isopened allowing the spring to extend and quickly return the foot to aneutral ankle position for swing phase.

FIG. 5 illustrates a plot of the damping values in each direction forthe damper during the gait cycle. In the direction of compression, thedamping during walking should be low enough to allow the foot to quicklyreach the surface, but not so low that the foot makes a slapping soundwhen it encounters the surface. The precise value will be dependent onthe weight, foot length, and gait mechanics of each individual user andwill parallel standard clinical practice for adjusting similarproperties of other commercially available components. Ideally theprecise amount of damping will be adjustable by the prosthetist forcustomization to the individual. For long-term standing tasks (doing thedishes, standing at a work station at work, etc.), an ideal embodimentwould also be able to raise the compression damping to near infinite(through the use of a cutoff valve) to improve stability when loadingthe heel, however this function is not of use during walking tasks. Theextension damping must be high or nearly infinite (closed cutoff valve,effectively fixing the length of the damper against extension) when theuser begins to roll over the foot (in the plot, this is shown asapproximately 5% of the gait cycle) and must remain so until the pointof toe-off. At the very beginning of swing phase, the cutoff valve isopened, allowing the damper to extend under the load from the springuntil the prosthesis has returned to a neutral position (ideally within0.13 seconds). After the foot has returned to a neutral position forswing phase, the cutoff valve can be closed again. Thus, the cutoffvalve could close as early as 0.13 seconds after toe-off or as late asat the moment of foot flat (approximated as 5% of the next gait cycle,but varies by step and surface conditions).

FIG. 6 illustrates a plot of the cylinder (damper) length. The currentmodel of the novel ankle-foot prosthesis has a damper with a fullyextended length of 70 mm, and a fully compressed length of 50 mm. Thiscorresponds to a 30-35 degree range of ankle motion. Other designs mayuse different numbers, however the relationships will still hold. Whenwalking on level ground, the ankle will plantarflex, allowing the footplate to become flat on the walking surface, during the firstapproximately 5-10% of the gait cycle. Once the foot is flat on thesurface and the user begins to roll over the foot, the damper is unableto extend, so the spring-damper combination remains at its partiallyextended length throughout the remainder of stance phase. When theprosthesis is lifted from the ground (TO), the cutoff valve is openedand the spring returns the foot to a neutral position for swing phase.When walking uphill, the foot will be flat on the surface in a moredorsiflexed position, so the foot will not plantarflex much during earlystance, whereas when walking downhill the foot will plantarflex muchmore before it is flat on the surface.

FIG. 7 shows three hydraulic circuit symbols used for fluid circuitschematics shown in FIGS. 8-16. The symbol used for the check valve 40is most commonly used to refer to a ball valve, although other types ofcheck valves may also be used. The variable restriction 42 is thedamping element of the circuit. There is some damping (fluid resistance)due to friction in the lines and passing through other elements of thecircuit, so there is a minimum level of damping regardless. Thus, insome embodiments, the restriction element is not present indicating theuse of innate damping alone. The reference numeral 44 designates acutoff value.

FIG. 8 discloses the most complex and powerful embodiment of the fluidcontrol circuit (FCC). The fluid circuit splits into two branches. Eachbranch has a check valve 40 oriented to permit fluid flow in eithercompression or extension alone, thereby separating the extension andcompression properties for the damper. In the compression line, there isprovided a variable restriction element 42, where the prosthetist couldadjust the damping level to optimize the prosthesis for the individualpatient. In the extension line, there is also a variable restrictionelement 42 that could be adjusted to tune the neutralization dampingafter toe-off to address any issues with the speed of neutralization orwith noises that could arise from underdamped neutralization. Both lineshave independent cutoff valves 44, allowing the extension damping to beraised to nearly infinite as appropriate during each step and then bothcutoff valves to be closed for standing tasks, making a stable base ofsupport for the user.

FIG. 9 discloses an embodiment that contains all of the adjustability ofthe embodiment of FIG. 8, but only a single cutoff valve 44 is used on acommon line to arrest both compression and extension of the dampersimultaneously. The advantage of this system over FIG. 8 is fewer parts(one fewer cutoff valve). The disadvantage of this system compared withthe embodiment of FIG. 8 is that sensors would need to be in place toinsure that the cutoff valve would open at the time of toe-off and closeat exactly the time of foot flat to prevent unexpected instability andpotential falls.

FIG. 10 discloses an embodiment that is similar to the embodiment ofFIG. 8, except it does not have a variable restriction element 42 on theextension line. Therefore, there is no way to tune the extension dampingfor neutralization after toe-off. This embodiment is more efficientbecause of the reduced number of components (saving weight, size andcost) but only if the fluid circuit can be optimized to allow the footto return to neutral within 0.13 seconds without oscillating or makingloud noises when it reaches the neutral position. This system retainsthe ability to cutoff both compression and extension for standing tasks.

FIG. 11 discloses an embodiment similar to the embodiments presented inboth FIGS. 9-10, however, it lacks the ability to adjust extensiondamping and has a single cutoff valve 44 to arrest motion in bothextension and compression simultaneously. This is even more efficient,having removed two components from the system and saving weight, size,and cost. The challenges with this embodiment have been discussed inparagraphs [0039] and [0040].

The embodiment of FIG. 12 is similar to the embodiment of FIG. 8, exceptthat it does not have a cutoff valve 44 in the compression line. Forthis reason, the compression damping will remain constant throughout thegait cycle and compression motion will not be arrested during standingtasks. Both lines have adjustable damping from the variable restrictionelements and the extension line still has a cutoff valve. Thisembodiment could be realized in a purely passive system, where thebiomechanics of walking (e.g. load on the prosthesis) control theopening and closing of the cutoff valve. For example, a spring-loadedhinge or telescoping element within the prosthesis could close thecutoff valve when load is applied to the prosthesis and open the cutoffvalve when load is removed from the prosthesis. It would not bepractical to rely on this type of physical input to control thecompression line for standing tasks, so none of the previous embodimentswould be practical for purely passive operation.

The embodiment of FIG. 13 is similar to the embodiments of FIGS. 10 and12, however, it also lacks the ability to adjust the damping in theextension line but saves weight, size, and cost. But it lacks theability to cutoff the compression line and therefore does not have thestanding stability feature of the earlier embodiments.

The embodiment of FIG. 14 is similar to the embodiment of FIG. 8, butlacks variable dampers 42. The level of resistance for compression canbe adjusted by the prosthetist by changing springs or by pre-compressingthe spring. Otherwise the function would be the same.

The embodiment of FIG. 15 is similar to the embodiment of FIG. 9, butlacks variable dampers 42. The level of resistance for compression canbe adjusted by the prosthetist by changing springs or by pre-compressingthe spring.

FIG. 16 shows our simplest embodiment. There is a check valve 40 topermit compression, but not extension, and then when the foot is to beneutralized the cutoff valve is opened, permitting extension bybypassing the check valve 40. Ideally the cutoff valve 44 would bemechanically opened and closed by loads applied during the gait cycle,resulting in a purely passive system with no batteries, microprocessors,or other electronic components, though this could be actuated by asolenoid or other actuator and controlled by electronics.

While this invention has been described as having preferred sequences,ranges, steps, order of steps, materials, structures, symbols, indicia,graphics, color scheme(s), shapes, configurations, features, components,or designs, it is understood that it is capable of furthermodifications, uses and/or adaptations of the invention following ingeneral the principle of the invention, and including such departuresfrom the present disclosure as those come within the known or customarypractice in the art to which the invention pertains, and as may beapplied to the central features hereinbefore set forth, and fall withinthe scope of the invention and of the limits of the claims appendedhereto or presented later. The invention, therefore, is not limited tothe preferred embodiment(s) shown/described herein.

REFERENCES

The following references, and any cited in the disclosure herein, arehereby incorporated herein in their entirety by reference.

-   1. Hansen, A., Childress, D., Miff, S., Gard, S., Mesplay, K. (2004)    The Human Ankle During Walking: Implications for Design of    Biomimetic Ankle Prostheses and Orthoses. Journal of Biomechanics,    Vol. 37, No. 10, 1467-1474.-   2. Williams R J, Hansen A H, Gard S A. (2009) Prosthetic Ankle-Foot    Mechanism Capable of Automatic Adaptation to the Walking Surface.    Journal of Biomechanical Engineering, Vol., 131, No. 3, 035002.-   3. Hansen A, Brielmaier S, Medvec J, Pike A, Nickel E, Merchak P,    Weber M (2012) Prosthetic Foot with Adjustable Stability and its    Effects on Balance and Mobility. 38th Annual Meeting and Scientific    Symposium of the American Academy of Orthotists and Prosthetists,    March 21-24, Atlanta. Ga.-   4. Nickel E A, Hansen A H, Gard S A. (2012) Prosthetic Ankle-Foot    System that Adapts to Sloped Surfaces. ASME Journal of Medical    Devices, Vol. 6, No. 1, 011006.

What is claimed is:
 1. An ankle-foot prosthesis, comprising: a) a footplate; b) an ankle frame attached to said foot plate; c) a yokepivotally connected to said ankle frame and including a member forattaching to a leg; d) a damper having a first end connected to saidyoke and a second end connected to said ankle frame; and e) a controlmechanism for switching said damper between low and high settings. 2.The ankle-foot prosthesis of claim 1, wherein: a) said damper comprisesa hydraulic damper for controlling extension damping.
 3. The ankle-footprosthesis of claim 2, wherein: a) said damper in the high settingprevents said yoke to pivot relative to said ankle frame.
 4. Theankle-foot prosthesis of claim 3, wherein: a) said control mechanismmaintains said damper at the high setting for at least about one-half ofa gait cycle.
 5. The ankle-foot prosthesis of claim 3, wherein: a) saidcontrol mechanism maintains said damper at the high setting for at leastabout first half of a gait cycle.
 6. The ankle-foot prosthesis of claim3, wherein: a) said control mechanism maintains said damper at the lowsetting between toe-off and mid-swing phases of a gait cycle.
 7. Theankle-foot prosthesis of claim 1, wherein: a) said damper comprises ahydraulic damper for controlling compression damping.
 8. The ankle-footprosthesis of claim 7, wherein: a) said control mechanism maintains saiddamper at the low compression damping substantially throughout a gaitcycle.
 9. The ankle-foot prosthesis of claim 1, wherein: a) said dampercomprises a hydraulic damper for controlling extension and compressiondamping.
 10. The ankle-foot prosthesis of claim 9, wherein: a) saidcontrol mechanism maintains said damper at the high extension andcompression damping for a standing mode.
 11. The ankle-foot prosthesisof claim 2, wherein: a) said damper in the low setting allows said yoketo pivot relative to said ankle frame.
 12. The ankle-foot prosthesis ofclaim 2, further comprising: a) a spring having a first end connected tosaid yoke and a second end connected to said ankle frame.
 13. Theankle-foot prosthesis of claim 12, wherein: a) said spring is disposedparallel to said damper.
 14. The ankle-foot prosthesis of claim 13,wherein: a) a change in the length of said spring is substantially equalto a change in the length of said damper.
 15. The ankle-foot prosthesisof claim 1, wherein: a) said ankle frame includes anterior and posteriorportions; and b) said anterior portion includes an upwardly inclinedsurface forming a roll-over surface with said foot plate.
 16. Theankle-foot prosthesis of claim 15, wherein: a) said inclined surface iscurved.
 17. The ankle-foot prosthesis of claim 16, wherein: a) saidposterior portion is pivotally connected to the second end of saiddamper.
 18. The ankle-foot prosthesis of claim 15, wherein: a) said footplate comprises a deflectable plate.
 19. The ankle-foot prosthesis ofclaim 15, wherein: a) said foot plate includes a forward deflectableportion.
 20. The ankle-foot prosthesis of claim 5, wherein: a) the gaitcycle comprises a walking mode.
 21. The ankle-foot prosthesis of claim1, further comprising: a) a fluid circuit operably connected to saiddamper; and b) said fluid circuit including at least one one-way valveand at least one two-way valve.
 22. The ankle-foot prosthesis of claim21, further comprising: a) at least one variable fluid-flow restrictorfor said fluid circuit.
 23. An ankle-foot prosthesis, comprising: a) afoot plate; b) an ankle frame attached to said foot plate and includinganterior and posterior portions and an apex portion; c) a yoke pivotallyconnected to said apex portion of said ankle frame and including amember for attaching to a leg; d) a hydraulic damper having a first endpivotally connected to said yoke and a second end connected to saidposterior portion of said ankle frame; e) a spring disposed in parallelto said damper; and f) a control mechanism for controlling extension andcompression of said damper.
 24. The ankle-foot prosthesis of claim 23,wherein: a) said control mechanism switches said damper between low andhigh settings to selectively control extension, compression, or bothextension and compression of said damper.
 25. The ankle-foot prosthesisof claim 24, wherein: a) said control mechanism maintains said damper atthe high extension setting for at least about one-half of a gait cycle.26. The ankle-foot prosthesis of claim 24, wherein: a) said controlmechanism maintains said damper at the high extension setting for atleast about first half of a gait cycle.
 27. The ankle-foot prosthesis ofclaim 26, wherein: a) said control mechanism maintains said damper atthe low compression setting during the entire gait cycle.
 28. Theankle-foot prosthesis of claim 26, wherein: a) the gait cycle comprisesa walking mode.
 29. The ankle-foot prosthesis of claim 24, wherein: a)said control mechanism maintains said damper at the low extensionsetting between toe-off and mid-swing phases of a gait cycle.
 30. Theankle-foot prosthesis of claim 24, wherein: a) said control mechanismmaintains said damper at the high extension and compression settings fora standing mode.
 31. The ankle-foot prosthesis of claim 23, wherein: a)a change in the length of said spring is substantially equal to a changein the length of said damper.
 32. The ankle-foot prosthesis of claim 23,wherein: a) said anterior portion of said ankle frame includes anupwardly inclined surface forming a roll-over surface with said footplate.
 33. The ankle-foot prosthesis of claim 32, wherein: a) said footplate comprises a spring plate.
 34. The ankle-foot prosthesis of claim33, wherein: a) said spring plate comprises a forward deflectableportion.
 35. The ankle-foot prosthesis of claim 23, further comprising:a) a fluid circuit operably connected to said damper; and b) said fluidcircuit including at least one one-way valve and at least one two-wayvalve.
 36. The ankle-foot prosthesis of claim 35, further comprising: a)at least one variable fluid-flow restrictor for said fluid circuit. 37.A method of using an ankle-foot prosthesis by an amputee, comprising thesteps of: a) providing an ankle-foot prosthesis, comprising: i) a footplate; ii) an ankle frame attached to the foot plate; iii) a yokepivotally connected to the ankle frame and including a member forattaching to a limb; iv) a damper having a first end connected to theyoke and a second end connected to the ankle frame; and v) a controlmechanism for switching the damper between low and high settings toselectively control extension, compression, or both extension andcompression thereof; b) attaching the ankle-foot prosthesis to a lowerlimb of the amputee; c) allowing the amputee to ambulate for at leastone gait cycle, the gait cycle comprising: i) the ankle-foot prosthesisin an initial neutral position to a first plantarflexion positionwherein the foot plate is substantially flat on a walking surface; andii) the ankle-foot prosthesis in a toe-off plantarflexion position; d)switching the damper to the high extension setting substantially at thefirst plantarflexion position; and e) switching the damper to the lowextension setting substantially at the toe-off plantarflexion position.38. The method of claim 37, further comprising the step of: f)maintaining the damper at the low compression setting substantiallythroughout the gait cycle.
 39. The method of claim 37, wherein: the gaitcycle comprises a second neutral position of the ankle foot prosthesissubsequent to the toe-off position; and the method comprises the step ofswitching the damper to the high extension damping substantially at thesecond neutral position.
 40. The method of claim 37, wherein: the gaitcycle comprises a standing or swaying position; and the method comprisesthe step of maintaining the damper at the high extension and highcompression settings during the standing or swaying position.
 41. Themethod of claim 37, wherein: the walking surface is substantially level,uphill, or downhill.